1. Field of the Invention
The present invention relates to an inkjet image forming apparatus and an image forming method. The image forming apparatus forms an image by discharging a recording liquid such as ink through a head.
2. Description of the Related Art
An image forming apparatus such as an inkjet printer has been known which includes a head that discharges a recording liquid such as ink through a plurality of nozzles and which performs inkjet recording (e.g., Patent Document 1 (Japanese Unexamined Patent Publication No. 2003-82265), Patent Document 2 (Japanese Unexamined Patent Publication No. 2003-246135), and Patent Document 3 (Japanese Unexamined Patent Publication No. 2000-343808)).
Since, during the inkjet recording, ink is discharged through a fine nozzle (e.g., several tens of micrometers), a highly soluble dye has been used as colorant of the ink to address a problem such as clogging of the nozzle. Dye ink is so excellent in color developing property that, for photographic printing, image quality of the dye ink is equivalent to that of silver halide photography. However, the dye ink is poor in image preservability, such as water resisting property, light resistance, or gas resisting property. To compensate for this problem, pigment has been used as colorant of ink. The pigment has been used for a large format printer for industrial use. Currently, the pigment is also used for a printer for personal use and a printer for office use.
When a color image is printed on a plain paper sheet, bleeding tends to occur in a color boundary, such as a two-color superposed portion, and feathering tends to occur in the vicinity of a printed portion, such as the vicinity of a character or a thin line. Techniques have been proposed to suppress such bleeding and feathering. In the techniques, a liquid or fine particles are used (e.g., Patent Documents 1, 2, and 3). For example, in Patent Document 1, an image forming technique has been proposed such that a process liquid including a polyvalent metal salt, which reacts with colorant included in ink and demonstrates a condensation effect, is utilized, and the ink is discharged onto a portion to which the process liquid is adhered. Further, for example, in Patent Document 2, an image forming technique has been proposed such that a process liquid including a cationic high molecular compound and a surfactant and/or a wetting accelerator is utilized, and ink is discharged onto a portion to which the process liquid is adhered. Further, for example, in Patent Document 3, a technique has been proposed such that a layer of water-absorbing resin fine particles (e.g., polyacrylic acid) is provided on a surface of an intermediate transfer body. In this technique, ink is applied onto the intermediate transfer body. The moisture of the ink is absorbed by the water-absorbing resin fine particles. Subsequently, the water-absorbing resin fine particles are transferred onto a recording medium together with the ink.
The technique which utilizes the process liquid including the polyvalent metal salt is effective for preventing the bleeding and the feathering. Unfortunately, when this technique is utilized, unevenness occurs within a dot. The technique which utilizes the process liquid including the cationic high molecular compound is effective for preventing the bleeding and the feathering. Unfortunately, since the cationic polymer is dissolved in the process liquid, the viscosity of the process liquid is high, and it is difficult to uniformly apply the process liquid. The application unevenness can be a cause of image distortion. Moreover, in this technique, the process liquid is an aqueous process liquid. When such a process liquid is applied to a plain paper sheet, curling and waviness tend to occur. Further, for a case in which the process liquid is applied to the intermediate transfer body, it is difficult to evenly apply the process liquid, as described above. Thus, the transfer efficiency is low, and the image density becomes low. In the technique in which the water-absorbing resin fine particles are utilized, excellent image quality is achieved even on a plain paper sheet, provided that a condition of the water-absorbing resin fine particles is good. Unfortunately, it is possible that, when the water-absorbing resin fine particles are stored, the water-absorbing resin fine particles absorb the moisture, and the water-absorbing resin fine particles are aggregated. In this case, it is difficult to uniformly apply the water-absorbing resin fine particles, and image distortion may be caused.
Accordingly, there is a need for an inkjet Image forming device and an image forming method such that they prevent feathering, bleeding, and curling, even if a plain sheet of paper is used as a recording medium, and such that they can form a high quality image by using a process liquid, which can be easily applied uniformly, and which can be relatively easily stored.